1. Field of the Invention
The present invention pertains to a solder stripping or tin stripper solution for stripping tin and/or tin-lead alloy from a copper or nickel substrate and, more particularly, pertains to a solder stripper solution for solder-mask-over-bare copper (SMOBC) type boards or for subsequent nickel-gold tab plating on printed circuit boards, nickel and/or nickel alloys, and copper or copper alloys.
2. Background of the Invention
The prior art solder strippers are divided into two types, the first type containing peroxide and the second type which is non-peroxide.
The peroxide type of solder strippers all have the same basic chemistry of a fluoride and a peroxide. One of the major problems with the peroxide system is that the reaction is very exothermic in that heat builds up rapidly during the stripping action. While tab stripping is not a particular problem, the loading factor must be reduced or cooling coils installed when stripping SMOBC boards. The peroxide type of solder strippers darkens the solder at the demarcation line because the mode of solder removal is more of a grain boundary attack than a true dissolving of the solder. Because of this factor, the solder turns very black with a white powdery residue. The strippers measle the epoxy since there is an immediate attack on the epoxy undercoat and as the solution heats up, the attack becomes even more pronounced. The solutions leave an insoluble white residue which becomes progressively thicker as more stripping is undertaken. Although the residue wipes off, it cannot be removed from holes or line overhang without a secondary treatment. Ventilation is always required in that the peroxide out-gasses from the solution emitting a mist of fluoride. As the solution exotherms, the out-gassing becomes more severe. Although the solutions have copper inhibitors, there is still a fair amount of copper attack. The rate of attack rises almost exponentially as the solution heats up. If boards with large areas to be stripped are put close together, the solution trapped between the boards can easily reach 135.degree.-150.degree. F., even though the main body may be at 85.degree. F. This trapped high temperature solution can lead to isolated, excessive copper attack which the stripper is unaware of.
The non-peroxide types have a more diverse chemistry than the peroxide type of solder stripper although most of the non-peroxide types utilize an inorganic acid. The non-peroxide strippers have definite advantages over the peroxide type strippers in that the non-peroxide strippers do not measle the epoxy, have controllable stripper rates and are only slightly exothermic, and truly dissolve the solder without simply undercutting the solder.
The present invention overcomes the disadvantages of the prior art by providing a non-peroxide stripper which is a one component system, clear and easily visible while stripping, and does not attack the solder, especially at the demarcation line and has an extended life span after the introduction of tin or solder alloy of weeks to months.
In one approach to overcoming the noted disadvantages of the prior art, U.S. Pat. No. 3,677,949 discloses a solder stripping composition consisting of a nitro-substituted aromatic compound, an inorganic acid selected from the group of fluoboric acid, fluosilicic acid, or sulfamic acid and thiourea. Additional additives were organic acid accelerators and wetting agents. This formulation has suffered since the useful life span is measured in hours after the introduction of tin or alloy thereof into the solution and there is a white powdery residual left on the surface of the base metal after stripping of the solder which required either scrubbing away or removal by a secondary solution. This patent is particularly ineffective for stripping tin or tin alloy thereof.
The present invention overcomes the disadvantages of the prior art by providing a non-peroxide solder stripping solution which is a one-component system, clear and visible with a straw-yellow color during stripping, does not attack the substrate or epoxy board, especially at the demarcation line, and has long shelf life.